cmd/go: allow use of Context in 'go list -f'

Add a $Context variable to the template so that the build.Context values
such as BuildTags can be accessed.

Fixes #6666.

LGTM=adg, rsc
R=golang-codereviews, gobot, adg, rsc
CC=golang-codereviews
https://golang.org/cl/72770043

src/cmd/go/context.go

+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"go/build"
+)
+
+type Context struct {
+	GOARCH        string   `json:",omitempty"` // target architecture
+	GOOS          string   `json:",omitempty"` // target operating system
+	GOROOT        string   `json:",omitempty"` // Go root
+	GOPATH        string   `json:",omitempty"` // Go path
+	CgoEnabled    bool     `json:",omitempty"` // whether cgo can be used
+	UseAllFiles   bool     `json:",omitempty"` // use files regardless of +build lines, file names
+	Compiler      string   `json:",omitempty"` // compiler to assume when computing target paths
+	BuildTags     []string `json:",omitempty"` // build constraints to match in +build lines
+	ReleaseTags   []string `json:",omitempty"` // releases the current release is compatible with
+	InstallSuffix string   `json:",omitempty"` // suffix to use in the name of the install dir
+}
+
+func newContext(c *build.Context) *Context {
+	return &Context{
+		GOARCH:        c.GOARCH,
+		GOOS:          c.GOOS,
+		GOROOT:        c.GOROOT,
+		CgoEnabled:    c.CgoEnabled,
+		UseAllFiles:   c.UseAllFiles,
+		Compiler:      c.Compiler,
+		BuildTags:     c.BuildTags,
+		ReleaseTags:   c.ReleaseTags,
+		InstallSuffix: c.InstallSuffix,
+	}
+}

src/cmd/go/doc.go

@@ -283,8 +283,7 @@ The default output shows the package import path:
 
 The -f flag specifies an alternate format for the list, using the
 syntax of package template.  The default output is equivalent to -f
-'{{.ImportPath}}'.  One extra template function is available, "join",
-which calls strings.Join. The struct being passed to the template is:
+'{{.ImportPath}}'. The struct being passed to the template is:
 
     type Package struct {
         Dir        string // directory containing package sources
@@ -332,6 +331,26 @@ which calls strings.Join. The struct being passed to the template is:
         XTestImports []string // imports from XTestGoFiles
     }
 
+The template function "join" calls strings.Join.
+
+The template function "context" returns the build context, defined as:
+
+	type Context struct {
+		GOARCH        string   // target architecture
+		GOOS          string   // target operating system
+		GOROOT        string   // Go root
+		GOPATH        string   // Go path
+		CgoEnabled    bool     // whether cgo can be used
+		UseAllFiles   bool     // use files regardless of +build lines, file names
+		Compiler      string   // compiler to assume when computing target paths
+		BuildTags     []string // build constraints to match in +build lines
+		ReleaseTags   []string // releases the current release is compatible with
+		InstallSuffix string   // suffix to use in the name of the install dir
+	}
+
+For more information about the meaning of these fields see the documentation
+for the go/build package's Context type.
+
 The -json flag causes the package data to be printed in JSON format
 instead of using the template format.
 

src/cmd/go/list.go

@@ -27,8 +27,7 @@ The default output shows the package import path:
 
 The -f flag specifies an alternate format for the list, using the
 syntax of package template.  The default output is equivalent to -f
-'{{.ImportPath}}'.  One extra template function is available, "join",
-which calls strings.Join. The struct being passed to the template is:
+'{{.ImportPath}}'. The struct being passed to the template is:
 
     type Package struct {
         Dir        string // directory containing package sources
@@ -76,6 +75,26 @@ which calls strings.Join. The struct being passed to the template is:
         XTestImports []string // imports from XTestGoFiles
     }
 
+The template function "join" calls strings.Join.
+
+The template function "context" returns the build context, defined as:
+
+	type Context struct {
+		GOARCH        string   // target architecture
+		GOOS          string   // target operating system
+		GOROOT        string   // Go root
+		GOPATH        string   // Go path
+		CgoEnabled    bool     // whether cgo can be used
+		UseAllFiles   bool     // use files regardless of +build lines, file names
+		Compiler      string   // compiler to assume when computing target paths
+		BuildTags     []string // build constraints to match in +build lines
+		ReleaseTags   []string // releases the current release is compatible with
+		InstallSuffix string   // suffix to use in the name of the install dir
+	}
+
+For more information about the meaning of these fields see the documentation
+for the go/build package's Context type.
+
 The -json flag causes the package data to be printed in JSON format
 instead of using the template format.
 
@@ -131,7 +150,18 @@ func runList(cmd *Command, args []string) {
 			out.Write(nl)
 		}
 	} else {
-		tmpl, err := template.New("main").Funcs(template.FuncMap{"join": strings.Join}).Parse(*listFmt)
+		var cachedCtxt *Context
+		context := func() *Context {
+			if cachedCtxt == nil {
+				cachedCtxt = newContext(&buildContext)
+			}
+			return cachedCtxt
+		}
+		fm := template.FuncMap{
+			"join":    strings.Join,
+			"context": context,
+		}
+		tmpl, err := template.New("main").Funcs(fm).Parse(*listFmt)
 		if err != nil {
 			fatalf("%s", err)
 		}

src/cmd/go/test.bash

@@ -694,6 +694,12 @@ unset go_cmds
 unset ldflags
 unset GOPATH
 
+TEST list template can use context function
+if ! ./testgo list -f "GOARCH: {{context.GOARCH}}"; then 
+	echo unable to use context in list template
+	ok=false
+fi
+
 # clean up
 if $started; then stop; fi
 rm -rf testdata/bin testdata/bin1